Measurement techniques for active pre-chamber ignition systems: optical experiments and thermodynamic evaluation

抄録

Homogeneous lean or diluted combustion can significantly increase the efficiency of SI engines. These diluted engine charges suffer from poor ignitability and lead to unstable combustion. Actively fuelled pre-chamber ignition systems can overcome this problem with a high power ignition from a chemical reaction inside the pre-chamber. The active fuelling ensures good ignition conditions inside the pre-chamber even with high EGR rates or lean cylinder charges.

The active fuelling is done by volatile components of gasoline fuel using an in-house developed fuelling system. This system is adapted to either high-EGR or lean combustion as the requirements for the scavenging systems are much different in these two operation modes. This actively scavenged pre-chamber ignition system adds more parameters like quality, timing and quantity of the scavenging gas to the engine, so the correct calibration of these additional parameters is mandatory to optimize the combustion process. The pre-chamber is designed to fit a miniature pressure transducer to evaluate the combustion process inside the pre-chamber. Together with the other pressure transducers that are commonly mounted in research engines, a thermodynamic analysis of the combustion process is possible. Thus, a 1D-simulation model can be calibrated to evaluate unmeasured parameters like exhaust gas content and fuel-air-ratio inside the prechamber at ignition timing.

To understand the combustion processes with active pre-chamber ignition systems, optical investigations of the flame luminescence are performed inside an optical accessible engine through a piston crown window. To record single cycles with high temporal resolution, a high speed intensifier and a high speed camera are used to reach an imaging frequency of 36 kHz. The selected engine speed is 1200 rpm. These measurements show the progress of the engine combustion through the cylinder under real engine conditions including charge motion.

To gain deep insight into the ignition process of a pre-chamber, more detailed optical investigations are required. These measurements are performed inside an optically accessible combustion vessel with constant pressure. To analyse the jet ignition process, a special insert to this constantly scavenged chamber is developed to create a reactive environment but maintain the constant pressure during combustion. For these measurements, high speed schlieren images and the OH*chemiluminescence are recorded simultaneously. With the superposition of these two optical techniques, the combustion progress can be visualized in detail. It reveals that the reactive jet from the pre-chamber shows only a very weak combustion signal, the main cylinder combustion is started by a secondary ignition inside these reactive jets. This visualisation of the combustion process without the influence of charge motion or pressure gradients enables to develop and calibrate a combustion model.

The combination of optical measurements and the thermodynamic analysis can show the major interrelationships of the pre-chamber ignition system. To develop a combustion system for passenger cars, the optimisation of the entire system from the geometrical design of the pre-chamber to the active scavenging system is necessary. The paper contribution demonstrates the co-working of modern diagnostics and modelling in the development of advanced combustion systems.